I remember the evening vividly. It was April 2001, and I was walking across the complicated system of roundabouts at Elephant and Castle in south London to meet a group of fellow activists in a bar at the Southbank. We were a group of anarchists, environmentalists and anti-capitalist protestors who were having a planning meeting for a May Day demonstration which was only days away. It was a balmy spring evening, and the sense of mounting excitement was palpable.

At these big meetings you'd see a group of your friends and gravitate towards them. These were people you shared a strong affinity with – people you'd been on big European protests with, who you'd put yourself at risk with, maybe even been arrested and beaten up with. These were people you trusted implicitly, and with whom you shared a strong bond.

When I arrived, one of the first people who grabbed me in an embrace was Rod. We had only been friends for a year or so but in that time shared a lot of intense experiences, living as we did in an environment of strong camaraderie and full-time activism. We had both been around various anti-capitalist groups where we had occupied buildings, worked together on actions, travelled around the country together and enjoyed long drinking sessions after protests. He would come and visit and we would have meals together; he would sit at the table with us discussing ideas and strategies. It wasn't until after he vanished without a trace in 2003 that I became suspicious that "Rod" wasn't whom he claimed to be, and that he may have been an undercover police officer.

At the time I was involved with various protest groups and was prominent on the London activist scene. Having grown up in Yorkshire I had been politicised by the miners' strike, and in the 1990s had been involved in protests against the criminal justice bill, environmental campaigns, squatting, and anti-capitalist actions against financial institutions in the City. The disparate groups that formed the "anti-capitalist movement" shared a sense of anger and indignation at the way unregulated markets were wreaking havoc on the environment globally. We believed that casino capitalism was the reason why so many people were living in poverty, and as an extension of this saw squatting as a political strategy to provide a practical solution to the crisis in homelessness.

As an artist, I was involved in symbolic street protests against the monarchy that were an attack on the British class system of privilege and hierarchy. The discussions we were having then, on the wealth of a ruling elite, have become even more pertinent 10 years down the line – the arguments we were having about the City, the banking industry and environmentalism have become standard points of view for a significant section of the British public. We discussed traffic disruption, guerilla gardening and, on one occasion, wheeling a guillotine around the Tower of London – we were not terrorists. Actions such as stopping the City, reclaiming the streets and transforming urban space in order to question the system we are living in is all work I still stand by politically.

After Rod's sudden disappearance, I began to thread together the disparate moments that had raised some suspicions. In 2001 we begun small meetings in my flat to discuss the logistics of blocking the Elephant and Castle roundabout to clog up a main artery into the City. Rod was a regular visitor, and even stayed the night there on occasion, most notably the night before the May Day 2001 protests. His reason for doing this was that he lived up in Hertfordshire and wanted to be in central London for the first actions of the day. Around this time, my flat was raided by the police – this seemed disproportionate when what we were actually arrested for was flyposting. We were held overnight in police cells, where even the duty sergeant expressed surprise that someone being held for "graffiti" would have their home raided.

When I was released from the police station at 5am and made my way back to the flat, it looked as if it had been burgled. They had ransacked everything. Getting a "visit" from the police is a violation – they had trampled through my bedroom turning everything over, ransacking cupboards, drawers and wardrobes. After they had gone I started to wonder if the phones were tapped.

On another occasion a friend and I went to Norwich to meet up with some people we had recently met, to discuss working together on some anti-monarchist actions. When we arrived at the house for dinner, which was a small affair of about eight people, I was really surprised to see Rod there as I wasn't aware he had any connection at all with these individuals or Norwich in general. I remember asking him how come he was there, and him being slightly affronted by the question. He shrugged and said he had been invited. It was the flash of discomfort on his face that triggered my suspicions. It just didn't seem right – there was a total sense of disconnect about his presence there. There were other things too, such as his claims to be an artist activist to explain sometimes bringing a video camera to actions. Strangely, none of the films he claimed he was making for Indymedia ever appeared there, but we now know who the real intended audience was.

Reflecting on it now I can't help feeling gullible, but at the time it seemed aggressive and paranoid to challenge every activist with a camera. Also, my feelings were just that – "gut feelings" are not enough to accuse someone of the worst thing you could ever accuse a fellow activist of being.

Later that summer I went into a deep depression. I thought that I was experiencing paranoid delusions in thinking that my phones were tapped and that people I had developed close friendships with might be police. This fear that the state might go so far in violating privacy, involving itself in the closest relationships and invading homes, made me nervous and anxious. For a few months I was a wreck, unable to really talk about it for fear I might seem completely unstable.

Ten years on, "Rod" is now suspected to have been an undercover police officer. It is a disturbing thing to read about, to know that the name we called him may actually have belonged to a baby who died at two days old.

Such an infiltration affects you psychologically, and impacts on your relationships with other people. It makes it more difficult to welcome new people into your friendship group. Politically, it's easy to see how damaging it is: the movement can't function if trust between activists is eroded. When a network is riven by accusations and suspicions, organisation and practical actions become an impossibility.

The weirdest thing of all is that I liked Rod a lot – he was such a nice bloke, always smiling and a good laugh in the pub. He appeared to be a committed activist, not afraid of breaking the law, challenging police lines and subjecting himself to, and in some cases instigating, difficult and dangerous situations for the sake of our collective principles. He never, to my knowledge, tried to initiate any kind of intimate relationship with anyone in the scene, but came across as genuinely decent and friendly. He left behind an odd floating feeling akin to grief, with questions left unanswered and a sense of betrayal and loss.

If I saw him now, I would for an instant expect the smile and the warm embrace, because I haven't adjusted to the idea that the entire friendship may have been fake. I am still deeply confused by the whole episode. There is an element of me that wonders if he experienced confusion as well. It's hard to accept that all those feelings of kinship and affection, those familial bonds that form through full-time activism, were perhaps a sham. If Rod was indeed misguiding us all along, surely feelings of revulsion and guilt must have shivered across him when we called out to him in that stolen name.